Two central functions of testosterone viz. modulation of hypothalamic LHRH levels (storage and secretion) and elicitation of male sexual behavior will be studied in rats. In the first project we will assess the role of endogenous opioid peptides in the steroid-dependent LHRH and LH responses, attempt to identify the opiate receptor subtype involved, and examine changes in the activities of hypothalamic catecholamines in response to testosterone and morphine treatment. In the second project, the role of preoptic dopamine producing neurons in mediating the central effects of testosterone on male sexual behavior will be examined by eliminating these dopamine projections, and with the use of specific dopamine agonists and antagonists in normoprolactinemic and hyperprolactinemic rats. Major techniques to be used include: stereotaxic application of drugs and opioid peptides in selected central sites; selective electrolytic lesions and deafferentations, radioimmunoassays of pituitary and hypothalamic hormones and steroid hormones; estimation of LHRH secretion rate in vivo by the push-pull cannula method and in vitro by the hypothalamic perifusion technique; radioenzymatic analysis of catecholamines turnover in hypothalamus and microdissected regions of the brain. These investigations should provide further insight into the neurochemical basis of testosterone modulation of reproduction and related behaviors.